Tuvok and Mousy Mick, it is definitely 20 minutes
20 printers 20 books 20 minutes
If you had 120 printers (6 times as many) and you still wanted to print 20 books, it would take you 1/6 of the time = 20/6 = 3 minutes 40 seconds
Now if you wanted to increase the volume of books required to 120, (6 fold) and you still had those 120 printers, it would take you 6 times as long ie 3 minutes 40 seconds x 6 = 20 minutes
If you had 120 printers and wanted to print 120 books it would take you 20 minutes.